An acute respiratory illness outbreak investigation at an Australian defence force training establishment

Background: Influenza outbreaks can spread rapidly in confined settings such as military training establishments, impacting operational capability. There are few published examples of influenza outbreak investigations in contemporary Australian military settings.Methods: An outbreak investigation was conducted in response to an increase in acute respiratory illness (ARI) cases presenting to an Australian military base health centre in June/July 2019. The investigation included a case test-negative analysis and an estimate of the 2019 influenza vaccine effectiveness in the outbreak population.Results: A total of 66 cases presented during the outbreak; 27 (40.9%) with confirmed influenza cases, 4 (6.1%) with suspected cases of influenza and 35 (53.0%) cases of non-influenza ARI. Those with confirmed influenza infection were significantly more likely to be from the main training unit on base, have a recorded fever over 38oC, and have not received the 2019 influenza vaccine. Cases of confirmed influenza also had significantly more time off work than those with non-influenza ARI. Vaccine effectiveness was estimated at 83% (95% CI = 42% to 95%), with an odds ratio of 0.17 (95% CI = 0.052 to 0.554) of confirmed influenza in those with 2019 influenza vaccination record compared to those without.Conclusion: This outbreak investigation reinforced the Australian Defence Force’s policy on influenza vaccination. It highlighted the impact that influenza can have on training and work capability, the need for ongoing outbreak surveillance and investigation, and areas for consideration in improving future outbreak control.